a) Field of the Invention
The invention is directed to a frequency-doubled diode pumped solid state laser with intracavity frequency doubling, comprising a lasing solid state medium with suitable reflecting coats which makes up a component part of a resonator cavity, at least one pumped light source constructed as a laser diode for generating suitable pumped light radiation, a frequency doubling nonlinear optical element in the form of a nonlinear crystal with suitable reflective coatings which forms another component part of the resonator cavity, wherein the resonator cavity is formed between reflective coatings of the solid state medium and of the frequency doubling element, and optical means following the pumped light source and conducting the pumped radiation to the lasing medium.
b) Description of the Related Art
Lasers of the type mentioned above have their own characteristic noise behavior which is brought about by the combined action of nonlinear processes such as frequency doubling, the mixture of longitudinal modes, and spatial hole burning in the active lasing medium. In lasers with internal frequency doubling, this characteristic is known as the "green problem". The noise behavior is characterized by a periodic change in the amplitude of the light by several tens of kilohertz to several hundred kilohertz or as a result of uneven fluctuations. The amplitude of the noise ranges from several percent to 100% of the modulation. Nonlinear crystals such as those used in solid state lasers exhibit absorption losses during radiation at the fundamental frequency and during frequency-doubled radiation. These absorption losses lead to unwanted heating of the nonlinear crystal. The changes in temperature occurring in this connection in the central zone of the laser beam can reach several hundred degrees K. As a result of the occurring temperatures and their distribution over the cross section of the nonlinear frequency doubling crystal, a detuning occurs at the latter, so that an optimum phase matching cannot be carried out resulting in reduced output of the frequency-doubled radiation. This means that almost no known frequency-doubled sold state lasers can operate in an optimum manner.
Various methods are known for eliminating or suppressing noise and for preventing these disadvantages.
For example, a method based on frequency selection and single-mode operation is described in the periodical "Optics Letters", Vol. 16 (1991), pages 1665ff ("Single-longitudinal-mode operation and second harmonic generation of Nd:YVO.sub.4 microchip laser"). Since there is no interaction between the longitudinal modes in single-mode operation, green noise is eliminated. Thus, there is only one oscillating mode and the noise caused by the green problem is virtually eliminated. The disadvantage therein consists in the relatively complicated construction of the resonator, whose object is to enable realization of single-mode operation (single longitudinal mode). In many cases, such low-noise operation of a laser of this kind is achieved through the use of a Type II nonlinear crystal (KTP or KTiOPO.sub.4) as part of a birefringent filter, e.g., a Lyot filter ("Optics Letters", Vol. 13, (1988), pages 805 to 807).
An intracavity frequency-doubled laser which is described in U.S. Pat. No. 4,933,947 has an improved amplitude structure which was achieved substantially in that the spatial hole burning in the lasing material is eliminated by the use of quarter-wave plates and the optical resonator of the laser is maintained at a temperature leading to an essentially low-noise generation of optical radiation. However, this requires a separate thermal stabilization of the arrangement depending on the noise level of the radiation.
German Offenlegungsschrift 42 05 587 A1 discloses a frequency-doubled single-frequency laser including a laser diode, a rare-earth-doped laser crystal, and a frequency doubling material in a shared resonator. This arrangement generates a stable green optical radiation and does not exhibit the problems caused by spatial hole burning, so that there is a single-frequency mode at the output of the laser.
A stable low-noise laser operation is achieved within a narrow temperature range which is realized by a suitable electronic circuit. However, a disadvantage consists in the complicated construction of the resonator which inhibits widespread use of this type of laser.
WO 95/21480 and U.S. Pat. No. 5,446,749 disclose a diode pumped laser in which the intracavity frequency doubling of a large number of longitudinal modes is used in order to suppress noise and achieve a high amplitude stability. The length of the resonator is configured for about 100 modes. This solution is disadvantageous in that it requires a resonator length of greater than one meter, which is not practical for many applications and limits its range of uses or application as a high-output laser.